A method of the aforesaid kind is known from ZELLERHOFF, M.; SCHOLZ, B.; RÜHRNSCHOPF, E.-P.; BRUNNER, T. “Low contrast 3D reconstruction from C-arm data”, Proceedings of SPIE, Medical Imaging 2005, Volume 5745, pages 646 to 655. With said known method, X-ray images of a patient are recorded from different projection directions with the aid of a C-arm that is provided with an X-ray source and an X-ray detector. The recorded X-ray images are corrected in respect of the scattered radiation taking into account an airgap. What is understood by airgap in X-ray imaging applications is the distance between the surface of the object being recorded and the detector. An evaluation unit connected downstream of the detector then produces volume images of the examined regions of the patient's body. Volume images are to be understood in this context as meaning three-dimensional images of the density distribution of the materials present in the body of the patient.
Flat-panel detector (FPD) technology allows low-contrast representation in the three-dimensional reconstruction of tissue density distribution by means of C-arm computed tomography. In the case of high-quality computed tomography systems having a fixed housing (=gantry) there is also an increasing trend to switch to two-dimensional multi-row detectors.
Because of the large patient volume that is irradiated when two-dimensional detectors are used, the scattered radiation containing only very little image information in each projection image represents a serious problem. The effects of scattered radiation are: loss of contrast, increased noise, and distortions of the quantitative results (“cupping” and artifacts) in the reconstructed images.
Measures to reduce scattered radiation are, as far as possible, the choice of a big airgap or the use of antiscatter grids. However, since for constructional reasons, inter alia, the airgap must be kept small and since in general the effectiveness of the antiscatter grid is inadequate in the case of FPDs, additional correction methods are necessary which, on the basis of mathematical estimations of the scattered radiation, are intended to compensate for its negative effects on image quality.
In DE MAN, B.; BASU, S.: Distance-driven projection and back-projection in three dimensions, Phys. Med. Biol. 49 (2004), pages 2463 to 2475, there is also a description of various projection methods, in particular a voxel-driven and a beam-driven method for forward projection (=reprojection) and back-projection.